1. Summary of the Invention
This invention relates to abrasive flow machining, and more specifically to a new and improved method and apparatus for abrasive flow machining utilizing at least one pair of extrusion chambers with the workpiece fixtured to but one of the chambers, from which an abrasive medium is unidirectionally extruded through the workpiece and upon exiting from said workpiece, the abrasive medium is permitted to fall into the other extrusion chamber. By subsequently fixturing a second workpiece to the second extrusion chamber, the abrasive medium is extruded in the reverse direction in a like fashion, falling back into the first extrusion chamber, where the sequence can be repeated.
2. Summary of the Prior Art
Abrasive flow machining is a well known nontraditional machining process whereby a visco-elastic medium, permeated with an abrasive grit, is extruded through or past a workpiece surface to effect an abrasive working of that surface. The abrasive action in abrasive flow machining can be thought of as analogous to a filing, grinding, lapping, or honing operation where the extruded visco-elastic abrasive medium passes through or past the workpiece as a "plug". The plug becomes a self forming, conforming to the surface of the workpiece as it is extruded under pressure through the confined passageway, thereby working the selected surfaces of the workpiece.
While abrasive flow machining is somewhat similar to other abrasion techniques wherein fluids are used as a medium to carry an abrasive grit in suspension for similar abrasion treatments, (such as hydrodynamic machining) there are considerable differences. In applications where fluids are used; i.e., liquids or gases, very high velocities are essential, not only to maintain the grit particles in suspension, but because high speed impingement of the grit particles against the surface to be abraded is the essential force in such processes. All such hydrodynamic machining processes are limited by the laws of fluid dynamics and are not, therefore, capable of uniformly machining complex surfaces.
In the present invention, as in other abrasive flow machining processes, however, the visco-elastic abrasive medium is a semi-solid plastic extruded through the restrictive passageway under considerable pressure but with a relatively low velocity. The semi-solid plastic medium not only maintains the abrasive particles in a uniform suspension, but it further provides a relatively firm backing for the abrasive grit to hold the grit firmly against the passageway surfaces while the semi-solid, visco-elastic medium and grit are extruded through or past the workpiece. Hence, rather than impinging at high speeds against the surface to be abraded, the grit slowly and actively works the workpiece surface with a much higher working force than a high velocity grit suspended in a fluid as it forcibly moves along the surface walls to be abraded. Unlike more conventional abrading techniques where the abrasive particles are held against the workpiece by a solid base support, however, the medium supporting the abrasive particles is plastic, so that as a backing material it will conform to the cross-sectional shape of the passageway, turning corners and changing shape as the passageway turns corners and changes shape.
The typical prior art apparatus utilized in abrasive flow machining consists of a structure holding two directly opposed extrusion chambers with the workpiece insertable therebetween. The extrusion chambers are plastic extruding, positive displacement, expandable chambers, such as mechanically driven piston displacement cylinders, which can extrude the abrading medium from one extrusion chamber through the passageway of the workpiece and then into the other extrusion chamber. One or two removable workpiece fixtures, designed to hold the workpiece and seal the workpiece passageway to the extrusion chambers, must be secured between the workpiece and the two extrusion chambers. The workpiece fixture must be designed to securely hold the workpiece such that the workpiece surface to be worked is exposed within the passageway between the two extrusion chambers to permit the abrasive medium to be extruded into and from the workpiece without any leaks. If a surface to be abraded is merely a bore through the workpiece, the fixture must serve to merely seal each end of the bore to an extrusion chamber so that the bore itself becomes a sealed passageway between one extrusion chamber and the other. On the other hand, if the workpiece surface to be abraded is an external surface, the fixture is usually more complex and must be designed so that the workpiece and fixture together define the essential restricted passageway so that the surface to be abraded forms a portion of the passageway, and the medium will abrade at least that surface as it is extruded through the passageway.
Some of the earlier techniques for abrasive flow machining were unidirectional processes which utilized one extrusion chamber from which the abrasive medium was extruded through an inlet fixture and through the workpiece passageway and then allowed to fall onto the machine table or into a container upon exiting the workpiece. At some point in time it became necessary to reload the extrusion chamber with the abrasive medium collected. Because of the extra effort and time involved in transferring the medium back into the extruding chamber, this unidirectional technique soon gave way to the more rapid bidirectional technique of extruding the abrasive medium back and forth through one or more workpieces (as described above) thereby eliminating the need to manually reload the single medium chamber and significantly shortening the overall processing time.
At the start of a cycle of operation, the extruding medium, consisting of a semisolid, difficulty flowable, visco-elastic material permeated with an abrasive grit, is contained in one of the extrusion chambers, while the other chamber is empty or near empty. To perform the process, the abrasive medium is extruded, hydraulically or mechanically, from the filled chamber to the empty chamber via the restricted passageway through or past the workpiece surface to be abraded, thereby working the surface as desired. Typically, the extruding medium is then extruded bi-directionally back and forth between the two extrusion chambers to the extent necessary to effect the degree of abrasion desired. Counterbores, recessed areas, and even blind cavities can be abraded by using restrictors or mandrils to direct and guide the abrasive medium flow along the surfaces to be abraded. A more detailed description of the basic prior art on abrasive flow machining can be found in U.S. Pat. Nos. 3,521,412, McCarty; 3,634,973, McCarty; 3,802,128Minear, Jr.; and 3,819,343, Rhoades.
While the prior art techniques are very effective, they do have their limitations with regard to certain workpiece characteristics. For example, some workpieces have complex geometries which make it difficult to design or apply fixtures that will effectively seal the opening to a passageway to be machined. As examples of such workpieces, some of the more advanced cylinder heads incorporating multiple intake and/or exhaust valves per cylinder are very difficult to fixture on both the manifold side and the piston cylinder side of the ports. In efforts to polish such intake or exhaust ports within such cylinder heads utilizing abrasive flow machining, it has been relatively easy to attach a fixture to the manifold side of the ports because the outer openings of the ports are usually located on a flat surface to which the intake or exhaust manifold will eventually be attached. The other ends of the ports, however, are not very easy to seal with a fixture because the port openings are normally very closely spaced within a domed or hemispherical cylinder head, which is further complicated by the fact that the dome will also contain a spark plug opening. While suitable fixtures can of course be designed, they are rather expensive to produce, and set-up time to properly mount the cylinder head workpiece to such fixtures can be rather time consuming if a seal adequate to prevent flow of the medium into areas such as exhaust ports and spark plugholes is to be achieved. In addition, reverse flow through such inlet ports does not work particularly well in most cases since the passageways are tapered in the opposite direction.